1. Field of the Present Invention
The present invention relates to a method for detecting and locating defects in an opaque layer of a semiconductor circuit. The present invention more particularly applies to the detection, the localization and the analysis of possible defects in a metallic track or a metallic polysilicide track of a multiple layer circuit including several levels of interconnects.
2. Discussion of the Related Art
Defects are likely to occur in metallic tracks of semiconductor circuits. These defects generally appear as a cross-sectional defect of the conductive track which modifies the current flow conditions for which the track has been provided.
The defects can have an electrical, mechanical or chemical origin. An electrical defect may, for example, be due to a displacing of the atoms under the effect of an electron flow caused by the current which flows through the track and generates the local heating of an area of the track which can cause its total or partial melting. Mechanical defects are, for example, due to thermal stress to which the circuit is submitted and which causes the diffusion or the extrusion of the metal. Chemical defects may, for example, be due to corrosion.
Several conventional methods of observation of possible defects in interconnecting layers present in a semiconductor circuit are known.
A first method consists in using an optical microscope to examine the semiconductor circuit. Such a method can only be applied to large structures (of several micrometers) for the detection of significant defects.
A second method consists in using a scanning electron microscope to examine the circuit. In a backscattered electron analysis mode, this method provides a large spatial resolution but it only applies to apparent layers. Thus, in the case of a multiple layer circuit, it is necessary to previously perform a pickling of the upper layers, for example by using chemical means or plasma, which may generate interpretation errors, and generate further defects or suppress existing defects in the circuit. The scanning electron microscope may be used in a potential contrast analysis mode, but this method requires the application of a potential to the metallic track, which may result in the closing of defects under the effects of an electric arc.
A third, so-called "thermal wave" method, consists in using a first laser (pulsed) for heating one point of the circuit and then surveying the point between two pulses of the laser with a second continuous low power laser. The survey measures variations in the reflectivity of the metallic track at the point under the effect of temperature. A measuring device enabling the implementation of such a method is, for example, commercialized by Thermawave under trade name Imager.RTM.. An advantage of such a method is that it has an important spatial resolution and is simple to use. However, in order not to be destructive, the power of the lasers must be limited (around 5 mW), which may limit the depth of the layers likely to be examined with respect to the surface of the circuit. The operating limit in depth generally is around 2 .mu.m. Yet, it is now more and more frequent to make multiple layer semiconductor circuits having a depth which can go up to around ten micrometers. Moreover, even in a non-destructive operating range of the laser, the pulsed heating-signal effect is dispersed for a buried semiconductor layer and thus the detection signal is strongly weakened, which leads to the variations of reflectivity being difficult to measure.
The third method is also used for the localization of defects in metallic polysilicide layers which behave in the same way as a metallic layer as concerns reflectivity to the laser beam. This method enables, for example, the detection of polysilicon layers which are poorly silicided.